The present disclosures relate to stereophonic audio encoders, and more particularly, to a NICAM encoding method.
Near-Instantaneously Companded Audio Multiplex (NICAM) encoding was developed during the early 1980's by the BBC research center. The main goals were to improve sound quality, provide multiple channels of digital sound or data, and improve ruggedness in difficult reception areas compared to other TV stereo systems, while preserving at the same time the compatibility with the existing services. NICAM 728 was first applied to the British TV system (PAL I) and later to PAL B/G and SECAM L. After examining several methods, in the late 1980's the ITU recommended the use of NICAM in countries using PAL and SECAM television systems for digital multisound transmission.
NICAM encoding is partly digital and partly analog. FIG. 1 is a schematic block diagram view of a prior art composite video with stereo audio system 10 having an analog filter 12, a dual-channel analog-to-digital converter (ADC) 14, a digital sound NICAM encoder 16, an analog QPSK transmitter 18, and an RF modulator 20. Analog filter 12 filters the two audio inputs 22 and 24, respectively, and outputs the filtered signals on outputs 26 and 28, respectively. The outputs 26 and 28 of analog filter 12 are inputs to the dual-channel ADC 14. ADC 14 receives a first clock at 34 (CLK1) and converts signals on the ADC inputs 26 and 28 into corresponding digital signals on ADC outputs 30 and 32, respectively. As illustrated, the outputs of the dual channel ADC 14 have 14-bit resolution. Digital sound encoder 16 receives a second clock at 38 (CLK2) and processes signals on encoder inputs 30 and 32 into digitally encoded signals on encoder output 36 according to the NICAM standard. Subsequently, the encoder output 36 is input to analog QPSK transmitter 18. QPSK represents Quadrature Phase Shift Keying. Analog QPSK transmitter 18 receives a third clock at 42 (CLK3) and QPSK modulates the signal received at the input 36 onto the output 40. The QPSK modulated signal on output 40 is then combined with the composite video on signal line 44 by RF modulator 20. The RF modulator then RF modulates the combined QPSK modulated signal and composite video onto RF modulator output 46.
Further in connection with the system of FIG. 1, pre-emphasis can be applied to the two inputs either in the analog or digital domain. The two input signals are digitized to 14 bit resolution at 32 kHz sample rate (CLK1) via ADC 14. The samples are grouped into blocks of thirty-two (32) 14-bit data, equivalent to a duration of 1 ms. At digital sound encoder 16, the samples of each block are companded to 10 bits with the same scaling factor. One parity bit is then added to each 10-bit sample for error detection and scale-factor signaling purposes. Left and right data are then multiplexed and bits are interleaved according to the interleaving pattern described in the NICAM standard, thus forming a block of 704 bits. Then an 8-bit frame alignment word, 5-bit control information, and 11-bit additional data are added at the beginning of the block of 704 bits, thus forming a frame of 728 bits. Each frame is serially transmitted every millisecond, for example, on signal line 36. The overall bit rate is 728 bit/s, corresponding to clock 38 (CLK2). The bitstream is then scrambled (except for the bits belonging to the frame alignment word), converted into two streams of 1-bit in-phase and quadrature data sampled at 364 kHz (symbol rate), differentially encoded and QPSK modulated, with use of clock 42 (CLK3), onto a 6.552 MHz subcarrier for PAL I or 5.85 MHz for PAL B, G and H and SECAM L via QSPK transmitter 18. The QPSK modulated audio signal 40 is then combined with the composite video 44 and RF modulated with RF modulator 20. The RF modulator produces RF signals 46 on VHF and/or UHF channels.
A disadvantage of the system of FIG. 1 is the requirement for multiple system clocks. That is, the NICAM encoder of FIG. 1 requires several clocks (e.g., CLK1, CLK2, CLK3, etc.) which are produced by different crystal oscillators and phase locked loops (PLLs). For example, for a dual-channel ADC that comprises a Sigma-Delta stereo ADC, the ADC is usually clocked at 4.096 MHz (corresponding to an oversampling rate of 128). The bit rate and symbol rate of the QPSK encoder are 728 kbit/s and 364 kbaud, respectively. The subcarrier frequency is 6.552 MHz for PAL I and 5.85 MHz for PAL B, G and H and for SECAM L. Note that these clocks are not easily related to one another, that is, they are not easily derived from a same clock, such as a 27 MHz clock or its multiples that are very common in audio/video chips or a 24 MHz clock, suitable for a single chip implementation, wherein the 24 MHz clock allows easy generation of a 4 MHz clock for the RF modulator integrated circuit. Furthermore, a disadvantage of requiring the use of PLLs is that PLLs require additional area and pins for ground and power supply. Moreover, since the crystal oscillators and PLLs are analog blocks, they are not easily portable. Accordingly, this adds extra complexity to the encoder and translates into additional overall cost.
Further as discussed above, a NICAM encoder is only partly digital. Some of its functions are implemented with analog blocks, in particular the pre-emphasis filters, the pulse-shaping filters of the QPSK transmitter and the QPSK modulator, which disadvantageously requires tuning and therefore adds considerable cost to the system. In addition, the direct implementation of these analog blocks into integrated circuits is not practical, because they are not easily portable when the technology is changed.
Still further, most of European television sets support NICAM to receive stereo audio from terrestrial television broadcasts wherever it is available. However, VCRs, DVD players, satellite set-top boxes and gaming stations are not equipped with NICAM encoders and therefore, if connected through the RF connector, only mono audio is available. Usually they are connected to television sets through a SCART connector (a.k.a. Euro connector). SCART stands for Syndicat francais des Constructeurs d'Appareils de Radio et de Télévision. Many consumer audio/video components in Europe support one or two 21-pin SCART connectors. The SCART connector has 21 pins and provides stereo sound and video signals both in and out of the equipment, depending on the particular type of equipment. In addition, the SCART connector may also provide RGB signals.
However, the SCART connector occasionally has reliability problems and sometimes, due to poor shielding, composite video output may interfere with the composite video input. Furthermore SCART cables can only be used to connect local equipment (e.g., located within the same room) and therefore cannot be used to connect remote television sets (e.g., located in distal rooms) to the set-top box. While newer European television sets are starting to include audio/video connectors similar to corresponding USA models, connecting several audio/video components, for example, to a European television set, through video, left and right audio cables can become complicated.
Traditional implementations of NICAM encoding systems are not very cost effective from the view point of integration into an audio/video chip or into a single-chip encoder due to the requirement of multiple clocks and the use of analog blocks which require tuning and which are not easily portable when integrated. NICAM encoders are generally used in TV stations and typically include very expensive rack mount units. While less costly versions may exist for other applications, the other applications still require a printed circuit board with many discrete components. Accordingly, in view of cost and complexity, NICAM encoders have been used mainly in broadcast equipment alone, and not in equipment for general consumer applications.
Accordingly, there is a need for an improved method for overcoming the problems in the art as discussed above.
The use of the same reference symbols in different drawings indicates similar or identical items. Skilled artisans will also appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.